Study On The Catalytic Performance Of Zn Modified ZSM-5 Zeolite For Pentane Aromatization

Aromatics are important raw materials for chemical products,mainly from catalytic reforming and steam cracking.Although the international aromatics market has been in a relatively low state,Chinese aromatics market still has a strong dependence on the import of other countries.Aromatization of light hydrocarbons not only can produce BTX aromatics,but also make reasonable use of light alkanes resources which have quite low value.However,the presence of these C₅-C₆ hydrocarbons in gasoline reduce the octane number to some extent,thereby their content have been seriously limited in the worldwide gasoline standards.However the reforming of pentane is not efficient,so the aromatization n-pentane has some practical significance.Owing to the excellent shape selectivity and high activity for aromatization,metal modified ZSM-5 zeolite has been widely used in the conversion of light alkanes to aromatics.The introduced metal species catalyze the dehydrogenation of alkanes,and the zeolite provides acid center.The effect of reaction temperature and WHSV on the n-pentane aromatization was investigated in a fixed bed reactor.The conversion of n-pentane increased with the rise of temperature.When the reaction temperature was too high,the cracking reaction was aggravated,which was not conducive to the formation of aromatics.The contact time between the reactants and the active center of the catalyst was relatively short in high WHSV,which had negative effect on the conversion of n-pentane.At the same time,the produced olefins directly left the reaction system,which could lead to the decrease of the aromatics yield.The intrinsic acid properties of zeolite are mainly determined by the Si/Al ratio of zeolite.The effect of Si/Al ratio on the aromatization of n-pentane was investigated in the experiment.Acidity is quite strong with low Si/Al ratio,which leads to direct cracking.When Si/Al ratio is relatively high,the cracking was decreased,but the hydrogen transfer and oligomerization were restrained.The results were that the selectivity of olefins increased,but the selectivity of aromatics decreased.The proper Si/Al ratio of ZSM-5 zeolite was about 38 for n-pentane aromatization.Further,the possible reaction pathways have been proposed through the distribution of products under different n-pentane conversion,and a reaction network has been built.Under the proper Si/Al ratio of ZSM-5 zeolite,the effect of the loading amount of Zn was investigated in the reaction of n-pentane to aromatics.When the load amount is low,too strong acidity led to low selectivity of aromatics though the conversion of n-pentane was high.Comparing with high loading amount of Zn,the conversion of n-pentane decreased,while the deactivation of zeolite was accelerated.The proper loading amount of Zn was about 2.0 wt%,and the highest selectivity of aromatics was about 32 wt%.Otherwise,the aromatization of methanol is an exothermic reaction,while the aromatization of n-pentane is endothermic.Considering the combination of methanol and n-pentane aromatization,the thermal coupling and synergistic effect were investigated.The experimental results showed that the reaction temperature of n-pentane aromatization decreased slightly,the selectivity of aromatics were increased and the selectivity of dry gas decreased.